The availability of adequate supplies of monoterpene aldehydes, particularly citral, is an issue of great importance to a number of industries. Although some processes for the commercial production of citral are available, new and/or improved synthetic routes are needed.
Organometallic chemistry has been used by a number of researchers in an attempt to effect various monoterpene transformations. Early experiments are reported in McQuillin et al., J. Chem. Soc. Perkin Trans. I, pp. 809-815 (1974), and Dunne et al., J. Chem. Soc. (C). pp. 2196-2200, 2200-2203, and 2203-2206 (1970). In these studies, the authors prepared several allyl palladium complexes of terpene compounds, including those resulting from the reaction of palladium with myrcene. Cyclization of myrcene, however, was found to occur, and citral was never obtained from the described processes.
Adding to this earlier work, Takahashi et al., Journal of Organometallic Chemistry, Vol. 266, pp. 327-336 (1984) successfully prepared a mixture of citral and nerol utilizing a two-step method. First, myrcene was reacted with dichlorobis(acetonitrile)palladium in the solvent hexamethylphosphoric triamide (HMPA) or in the presence of a base such as Li.sub.2 CO.sub.3 using dimethylformamide (DMF) as solvent, to yield a non-cyclized palladium-myrcene complex. Although the reported yield for the HMPA process was relatively good, the Li.sub.2 CO.sub.3 /DMF process yield was somewhat low, approximately 33%. In the second step of the reported process, the complex was isolated, and then treated with base to yield terpene aldehydes and alcohols such as citral and nerol. One major drawback of these processes, however, is that they necessitate two steps, requiring isolation of the intermediate before further processing. Moreover, the product obtained using these methods is a mixture of both citral and nerol. Furthermore, the reactions are saddled with the added disadvantage of a temperature limitation, since at temperatures above about 60.degree. C. the solvents HMPA and DMF are decomposed by the palladium species. See Bombieri et al., Inorganica Chimica Acta, Vol. 86, pp. 121-125 (1984); Fahey et al., Journal of Organic Chemistry, Vol. 39, pp. 3276-77 (1974). In addition, the use of HMPA in this, or any process, is clearly undesirable, since HMPA is an extremely potent toxin, as well as a suspected carcinogen.
Citral is a compound of high significance to the flavor, fragrance and synthetic vitamin industries. Additional and/or better processes for their commercial production, particularly processes employing the readily available starting material myrcene, are needed. The present invention is directed to this very important end.